As means for diagnosing diseases, generally used are methods for detecting a causative substance (for example, antigen) existing in a test sample, for example, fluid such as serum and urine, culture broth, extract, feces or the like or its related substance (for example, antibody). Conventionally, amount of a test substance such as an antibody or antigen in a test sample is measured by utilizing a biologically specific reaction such as an antigen-antibody reaction. As a kind of such detecting methods, the solution development method (chromatographic method) including an immunochromatographic method has been proposed heretofore (The Medical & Test Journal, No. 706, p.5, Oct. 11, 1999).
The solution development method is a technique in which a sample is applied to a part of a test piece and penetrated and developed in the test piece material usually by using a developing liquid such as water to detect a test substance existing in the sample. In the immunochromatographic method, which is one type of the solution development method, an immune complex formed between a test substance and a substance corresponding to the test substance is detected in a test region by using a labeled reagent.
In the solution development method, in order to confirm whether a developing liquid containing a test substance penetrates into a test region of the test piece and the development is completed, that is, completion of the test, the test piece can be provided with a specific site called a development completion confirming region. A development completion signal is a requirement to be satisfied by the test system upon determination of a test result, that is, it plays a role of presenting the fact that “a specific amount of developing liquid has passed through a test region together with a labeled reagent over a certain time, and has further reached the development completion confirming region”, and is a test completion signal for a practitioner of the test. Confirmation of the signal is important to make the test result reliable. For example, it is important for determining that the test result is negative (no reaction is observed) or the like.
As a method for confirming a development completion signal, there are known a method of adding a substance that undergoes an antigen-antibody reaction into the development completion confirming region, a method of adding an indicator that develops color or changes in its color in response to a change in pH into the development completion confirming region and so forth.
Further, the solution development method has been utilized as a qualitative determination method because of its detection accuracy.